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Heater blowing cold air

2005 gmc 4.3 heater blowing cold air

Heater blowing cold air

Postby Esear73 [OP] » Sep 28 2020, 8:07pm

I have a 2005 gmc the heater is blowing cold air on both sides. It started blowing cool air last year so I changed the TS it started working ok then. This year it’s back to cold air only a/c works great. With the information from this form I gather it’s the actuator or air in core. This gmc has four actuators which one would control the temperature.
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Re: Heater blowing cold air

Postby Walkers » Sep 28 2020, 9:35pm

Feel the hoses to see if they are hotbefore you do anything else.
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Re: Heater blowing cold air

Postby Esear73 [OP] » Sep 29 2020, 12:32am

The hose going into top of block is hot. The hose going to the water pump or just below it is cool.
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Re: Heater blowing cold air

Postby ricka182 » Sep 29 2020, 2:42pm

Do you get heat when you accelerate? Could be a blocked heater core, which is pretty much worst case. You'd have to remove most of the dash and trim to access it.
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Re: Heater blowing cold air

Postby Walkers » Sep 29 2020, 3:47pm

Sounds like a blocked heater core. Pull the hose off and see it you can get water from a garden hose through one direction or the other.
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Re: Heater blowing cold air

Postby Esear73 [OP] » Sep 30 2020, 2:02am

Do you have to reverse flow it or does it matter?
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Re: Heater blowing cold air

Postby ricka182 » Sep 30 2020, 5:58pm

You can do both directions, in case something is blocking fins on one side or the other. Be sure to use a lower pressure, or you could do more damage. Sometimes that little flush may dislodge something and allow fuller flow...other times it may break down corroded areas and make the heater core leak...so check the passenger side floor and keep an eye on smell as well.
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Re: Heater blowing cold air

Postby CT2500HD » Sep 30 2020, 8:48pm

Every time this happens, the first thing I do is turn-up the fan speed to max, then change the temp to the other direction.
I'm not sure the exact parts that get happy where they are for the duration of the summer or winter, but I've had this happen I swear to goodness all I do is turn up the fan speed, then change the temp and it flips-over.
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